Apparatus and method for controlling temperatures of hot gas lift for pebbles



M y 1 1956 L. P. MEADE APPARATUS AND METHOD FOR CONTROLLING TEMPERATURES OF HOT GAS LIFT FOR PEBBLES Filed June 11, 1951 INVENTOR.

L P MEADE FEED ATTOP [VS United States Patent APPARATUS AND METHOD FOR CONTROLLING gugr ramrunns OF HOT GAS LIFT FOR PEB- Leonard P. Meade, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Application June 11, 1951, Serial No. 230,865

7 Claims. (Cl. 263-19) This invention relates to the lifting of pebbles by a gas lift in a system of the pebble heater type. In one of its aspects it relates to avoiding excessive thermal shock to the pebbles.

Pebble heater apparatus is finding increasing favor in effecting chemical reactions continuously at temperatures in the range of 1500-3500 F., and in some instances, as high as 4000" F. It has been found that pebble heater apparatus is particularly well adapted to effecting chemical reactions at exceedingly high temperatures and short reaction times. Some of the factors which favor the utilization of pebble heaters are the extremely sharp heating rates possible with this type of apparatus, and the avoidance of contamination of the reaction products with combustion gases. Another factor is that sensible heat from the efiluents from the reactor, or from the stack gases leaving the heating chamber, may be utilized to provide process steam.

In a number of the commercial pebble heaters in operation the lifting means is comprised of a mechanical elevator which takes pebbles from the feeder below the reactor and elevates them to the top of the heating chamher. As stated, some processes are carried out at exceedingly high temperatures and the temperature of the cooled pebbles is still above the efiective operating range of mechanical elevators. and necessary to employ a gas lift. However, gas lifts are now being employed more widely in pebble heaters because of their simplicity and low maintenance costs. The gas for the lift is usually supplied by an air blower.

The term pebble as used herein denotes any solid material of fiowable form and size which can be used to carry heat from one chamber or zone to another. Substantially spherical pebbles of about inch to inch in diameter can be used in most instances, but the preferred size in most processes lies between 1 inch and /2 inch diameter. Pebbles of alumina, beryllia, magnesia, thoria, zirconia, mullite, periclase and other materials, preferably refractory ceramic materials, are suitable for various pebble heater processes. In pebble heater operation it is necessary to select pebbles which will withstand the extremely high temperature and the shock conditions of temperature change, impact and abrasion to which they are subjected in the apparatus. Hard, dense, glazed pebbles compacted from relatively pure alumina and fired at about 3000 F. will stand up well over long periods in pebble heater operation.

A problem encountered in pebble heater operation is thermal shock to the pebbles. Thermal shock results in a shorter life for the pebbles and usually manifests itself in splitting or reduced resistance to crushing and abrasion. Broken pebbles or lines have a definite tendency to bridge or form clinkers which can plug the apparatus and necessitate a costly shut down. Hence, it can be seen that thermal shock can become a serious problem in operating systems of the pebble heater type in which compact masses of particles must be kept con- In such situations it is desirable,

tinuously gravitating and flowing. When hot pebbles are contacted with cool gases under conditions such that a relatively large temperature difference exists, the rate of transfer of heat outward from the hot center to the relatively cool surface of the pebble may not be suflicient to prevent thermal strains from rupturing the pebbles. The two factors of greatest importance are pebble diameter and temperature difference. The smaller the pebble the less is the time required to equalize temperature gradients, and in processes utilizing solid spheres of a. diameter less than about Ms inch, e. g. fluidized solids systems, the problem is relatively unimportant. In pebble heater systems, however, other considerations make it preferable to use larger sized spheres. Thus, for example, spheres of A to /2 inch in diameter are preferred over smaller sizes because higher countercurrent flow rates can be employed without fluidizing the bed. When these larger sized particles are employed the factor of temperature difference must be controlled in order to prevent damage from thermal shock.

Pebbles normally suffer some thermal shock in the reactor when the feed first contacts the hot pebbles. This is overcome, however, to some extent by the system itself. That is to say, in the countercurrent flow of pebbles and feed, the coldest pebbles contact the coldest feed. The problem of thermal shock is greatest in the concurrent flow of pebbles, as in the gas lifting means. The present invention is directed to the prevention or minimizing'of thermal shock in the lifting means of the apparatus.

It is an object of this invention to provide an improved method and apparatus for elevating pebbles in a pebble heater type system.

A further object of this invention is to provide a means for avoiding excessive thermal shock to pebbles in the gas lifting means.

Other objects and advantages of this invention will become apparent from the accompanying disclosure and discussion.

The accompanying drawing portrays diagrammatically one arrangement which can be employed for carrying out the present invention. Illustrated is an ordinary pebble heater unit which can be used for a cracking process such as the cracking of ethane or propane. This operation usually takes place at a temperature near 1500 F. Pebbles are heated in heating chamber 10 by the combustion gases from burner 11. The temperature can be controlled to the desired level by conventional control devices. One method may be to control the amount of combustion air through valves 12 or 13 in response to a thermocouple in pebble duct 15, or valve 14 in the combustion fuel line can be controlled in the same manner. The hot pebbles drop down through duct 15 into reaction chamber 16 where they contact the feed material to be cracked. Seal steam is admitted into duct 15 through line 20 to prevent leakage of combustion gases into the reaction chamber despite differences in pressure between the two chambers. The feed gas enters the reaction chamber through line 21 and effiuents leave through line 22 and pass on to additional treatment not shown in the diagram. The cooled pebbles leave the bottom of the reaction chamber and pass through conduit 25 to pebble feeder 23 where they are fed at a constant rate into the bottom of pebble lift 26. The pebbles are usually at a temperature of 900 F. or higher when they enter the feeder. Seal steam is admitted into conduit 25 via line 24 to prevent leakage from the reaction chamber. The lifting of the pebbles is accomplished by means of blower 30 to provide the proper gas velocity and burner 31. In a preferred embodiment of the invention, the temperature of the lifting gas is automatically controlled by means of a conventional differential temperature recorder controller 3 32 which records the temperatures of the pebbles entering the lift and the lift gas. This controller actuates motor valve in the fuel supply line to burner 31 in response to a predetermined difference between pebble and lift gas temperatures. This difference is preferably zero and a maximum of 50 F. A method of operation of this invention which has proved satisfactory is to have the primary air aspirate the fuel and effect temperature control of the lift gas in that manner. However, it is a matter of choice whether to control valves 33, 34 or 35. Valve 35 is preferred. Broadly, any means, such as heat exchangers, which may be employed to adjust the temperature of the lift gas to essentially that of the pebbles entering the lift are Within the scope of my invention. It is also within the scope of my invention to use the combustion gases leaving the heating chamber as the lift gas.

An added advantage to using a burner between the blower and the lift is that the volume of lift gas is increased without an increase in the amount of air which blower 30 must handle. Using ordinary unheated air as the lifting gas results in excessive thermal shock to the pebbles and results in fracture or decreased resistance to crushing and abrasion. Also, the pebbles are further cooled and require a longer residence time in the heating chamber thereby cutting down the rate of flow of hot pebbles into the reaction chamber, or necessitating the use of a greater amount of pebbles in the system to compensate for the increased residence time.

The pebbles entering the bottom of lift 26 are elevated to the pebble settling chamber where the fines which are lifted to the top of the chamber are separated by means of a baffle and fall into fines removal drum 41. Sensible heat from the lift gas can be utilized for various purposes, such as providing process steam.. The pebbles from the bottom of settling chamber 49 flow through pebble conduit 42 and into the top of heating chamber 10' where they are again heated and recycled. An optional method for fines removal can consist of passing the entire amount of material from the settling chamber over a screen built into pebble conduit 42.

Although the concept of my invention has been described and exemplified in terms of its preferred modifications, it will be understood that various changes may be made without departing from the spirit and scope of the disclosure and of the claims.

I claim:

1'. In elevating pebbles by a gas lift in a pebble heater system wherein the pebbles enter said gas lift at a predetermined temperature, the improvement which comprises controlling the temperature of the lift gas' so that it is about equal to the predetermined temperature of the pebbles entering said gas lift.

2. In elevating pebbles by a gas lift in a pebble heater system wherein the pebbles used are at least 0.3 inch in diameter and enter said gas lift at a predetermined temperature, the improvement which comprises controlling the temperature of the lift gas so that it is about equal to the predetermined temperature of the pebbles entering said gas lift.

3. In elevating pebbles by a gas lift in a pebble heater system wherein the pebbles used are at least 0.3 inch in diameter and enter said gas lift at a predetermined temperature, the improvement which comprises burning fuel with air in a combustion Zone; mixing lift gas with the resulting combustion products; and controlling the amount of fuel burned in said combustion zone in response to the difference in pebble temperature and the temperature of the mixture of lift gas and combustion products so that the temperature of said mixture is about A equal to the predetermined temperature of the pebbles entering said gas lift.

4. The improvement in accordance with claim 3 in which the amount of fuel burned in said combustion zone is controlled so as to maintain the difference in pebble temperature and the temperature of the lift gas and combustion products in the range of 0 to 50 F.

5. A process which comprises gravitating a compact mass of pebbles having a diameter of at least 0.3 inch through at least a pebble heating zone and a heat transfer zone; continuously Withdrawing a stream of said pebbles at a predetermined temperature from the bottom of said heat transfer zone; entraining said pebbles so withdrawn in an up-flowing stream of lift gas; controlling the tem perature of said lift gas so that it is about equal to the predetermined temperature of said pebbles; separating said pebbles from said lift gas at a point above said pebble heating zone; and passing said separated pebbles into said pebble heating. zone.

6. An improved pebble heat exchange system comprising, in combination, a first closed, upright, pebble chamber having pebble inlet means and gaseous outlet means in its upper end portion and fluid inlet means in its lower end portion; a second closed, upright pebble chamber disposed below said first chamber and having gaseous outlet means in its upper end portion and fluid inlet means and pebble outlet means inits lower end; pebble conduit means connecting the lower end of said first chamber and the upper end of said second chamber; a gas-pebble separator chamber connected to said pebble inlet means; an upright gas lift conduit connected at its upper end to said gas-pebble separator chamber and connected at its lower end to said pebble outlet means; lift gas inlet means connected to the lower end of said gas lift conduit; means for detecting the temperature differential between that of the pebbles flowing. through said pebble outlet means and that of the lift gas in said lift gas inlet means; and means for controlling the temperature of the lift gas in response to said detected temperature differential.

, 7. An improved pebble heat exchange system comprising, in combination, a first closed, upright pebble chamber having pebble inlet means and gaseous outlet means in its upper end portion and fluid inlet means in its lower end portion; a second closed, upright pebble chamber disposed below said first chamber and having gaseous outlet means in its upper end portion and fluid inlet means and pebble outlet means in its lower end; pebble conduit means connecting the lower end of said first chamber and the upper end of said second chamber; a gas-pebble separator chamber connected to said pebble inlet means; an upright gas lift conduit connected at its upper end to said gas-pebble separator chamber and connected at its lower end to said pebble outlet means; a lift gas inlet connected at one end to the lower end of said gas lift conduit; a burner having its outlet end connected to the other end of said lift gas inlet conduit; gas inlet means connected to said burner; fuel inlet means connected to said burner; a differential temperature control means operatively connected to said pebble outlet means and to said lift gas inlet conduit; and a flow control means positioned in said fuel inlet means and operatively connected to said differential temperature control means.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,759 Angell Apr. 23, 1946 2,432,873 Ferro et a1. Dec. 16, 1947 2,614,028 Schaumann Oct. 14, 1952 

6. AN IMPROVED PEBBLE HEAT EXCHANGE SYSTEM COMPRISING, IN COMBINATION, A FIRST CLOSED, UPRIGHT, PEBBLE CHAMBER HAVING PEBBLE INLET MEANS AND GASEOUS OUTLET MEANS IN ITS UPPER END PORTION AND FLUID INLET MEANS IN ITS LOWER END PORTION; A SECOND CLOSED, UPRIGHT PEBBLE CHAMBER DISPOSED BELOW SAID FIRST CHAMBER AND HAVING GASEOUS OUTLET MEANS IN ITS UPPER END PORTION AND FLUID INLET MEANS AND PEBBLE OUTLET MEANS IN ITS LOWER END; PEBBLE CONDUIT MEANS CONNECTING THE LOWER END OF SAID FIRST CHAMBER AND THE UPPER END OF SAID SECOND CHAMBER; A GAS-PEBBLE SEPARATOR CHAMBER CONNECTED TO SAID PEBBLE INLET MEANS; AN UPRIGHT GAS LIFT CONDUIT CONNECTED AT ITS UPPER END TO SAID GAS-PEBBLE SEPARATOR CHAMBER AND CONNECTED AT ITS LOWER END TO SAID PEBBLE OUTLET MEANS; LIFT GAS INLET MEANS CONNECTED TO THE LOWER END OF SAID GAS LIFT CONDUIT; MEANS FOR DETECTING THE TEMPERATURE DIFFERENTIAL BETWEEN THAT OF THE PEBBLES FLOWING THROUGH SAID PEBBLE OUTLET MEANS AND THAT OF THE LIFT GAS IN SAID LIFT GAS INLET MEANS; AND MEANS FOR 